U.S. patent application number 13/361819 was filed with the patent office on 2012-08-16 for shape measuring apparauts and shape measurieng method.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Ryusuke Nakajima.
Application Number | 20120204435 13/361819 |
Document ID | / |
Family ID | 46620634 |
Filed Date | 2012-08-16 |
United States Patent
Application |
20120204435 |
Kind Code |
A1 |
Nakajima; Ryusuke |
August 16, 2012 |
SHAPE MEASURING APPARAUTS AND SHAPE MEASURIENG METHOD
Abstract
A contact type prove achieves a high-precision measurement of a
shape of even a steeply-inclined surface in the vicinity of
vertical by controlling a contact force stably. In a shape
measuring method for measuring the shape of the surface of a
measured object by moving a contact type probe along the surface of
the measured object, a slope of the measured object surface is
estimated depending on the magnitude of a component force of the
contact force applied to the probe and when it is determined that
the slope is in the vicinity of vertical, a probe supporting unit
is moved in a direction perpendicular to the moving direction of
the probe supporting unit.
Inventors: |
Nakajima; Ryusuke;
(Kawasaki-shi, JP) |
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
46620634 |
Appl. No.: |
13/361819 |
Filed: |
January 30, 2012 |
Current U.S.
Class: |
33/503 |
Current CPC
Class: |
G01M 11/00 20130101;
G01B 5/20 20130101; G01B 5/008 20130101; G01M 11/025 20130101 |
Class at
Publication: |
33/503 |
International
Class: |
G01B 5/008 20060101
G01B005/008 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 14, 2011 |
JP |
2011-028716 |
Claims
1. A shape measuring apparatus for measuring a shape of a measured
object by scanning a contact type probe along the surface of the
measured object while keeping the contact type probe in contact
with the measured object and measuring a position of the contact
type probe, the shape measuring apparatus comprising: a probe
supporting unit configured to be movable in a three-dimensional
direction; a contact type probe configured to be supported
elastically with respect to the probe supporting unit; a measuring
unit configured to measure the position and orientation of the
contact type probe; and a calculation unit configured to calculate
a contact force received by the contact type probe from the
measured object based on a measured position and orientation of the
contact type probe, wherein the probe supporting unit is
configured, when a component force containing a component in a
moving direction of the probe supporting unit of the contact force
exceeds a predetermined threshold, to be driven in a direction
perpendicular to the moving direction of the probe supporting unit
to move the contact type probe.
2. The shape measuring apparatus according to claim 1, wherein the
probe supporting unit is configured to be driven by providing the
probe supporting unit with a speed or an acceleration proportional
to a difference between the component in the moving direction of
the probe supporting unit and the threshold.
3. The shape measuring apparatus according to claim 1, wherein the
probe supporting unit further comprises: a slide movable in a
3-dimensional direction; and a table supported by the slide movably
in a 3-dimensional direction with respect to the slide, wherein,
the contact force magnitude is configured to be brought close to a
target value by moving the table to move the position of the
contact type probe.
4. A shape measuring method for measuring the shape of a measured
object by scanning a contact type probe supported elastically by a
probe supporting unit movable in a 3-dimensional direction along
the surface of the measured object while keeping the contact type
probe in contact with the measured object and measuring a position
of the contact type probe, the shape measuring method comprising:
measuring the position and orientation of the contact type probe
and calculating a contact force based on the measured position and
orientation of the contact type probe; scanning the surface of the
measured object with the contact type probe by moving the probe
supporting unit while controlling the contact force to be brought
close to a target value with a force control unit; and driving the
probe supporting unit in a direction perpendicular to the moving
direction of the probe supporting unit to move the contact type
probe, when a component force containing a component in a moving
direction of the probe supporting unit of the contact force exceeds
a predetermined threshold.
5. The shape measuring method according to claim 4, wherein the
probe supporting unit is configured to be driven by providing the
probe supporting unit with a speed or an acceleration proportional
to a difference between the component in the moving direction of
the probe supporting unit and the threshold.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a shape measuring apparatus
having a stylus type probe capable of measuring a surface shape of
an optical element such as a lens and a mirror, and a mold for
manufacturing the optical device at a high precision on the order
of nanometer, and a shape measuring method thereof. More
particularly, the present invention relates to a shape measuring
apparatus capable of coping with measurement of a shape having a
steep inclination angle, for example, a shear wall surface rising
vertical from horizontal.
[0003] 2. Description of the Related Art
[0004] Generally, as a shape measuring method for measuring
coordinates or a shape of a specific portion of the surface of a
measured object having a 3-dimensional shape, a measuring method
using the stylus referred to as probe has been known. According to
this measuring method, the probe is allowed to trace the surface of
the measured object while pressed against the surface of the
measured object at a predetermined contact force, so that, by
measuring a position of the probe with respect to a predetermined
origin and an orientation of the probe, the shape of the measured
object is measured.
[0005] Conventionally, as regards such a shape measuring method, a
contact type probe based on a shape measuring method as discussed
in Japanese Patent Application Laid-Open No. 2005-37197 has been
known to those skilled in the art. According to this method, a
stylus probe is used to measure the shape of a measured object, the
probe being supported with a leaf spring suspended from a housing.
The probe is provided with a displacement sensor capable of
measuring a relative position of the probe with respect to the
housing. Based on these sensors and preliminarily measured
stiffness (spring constant) in each direction of the suspended leaf
spring, a contact force can be measured based on a displacement
generated when the probe comes into contact with the measured
object. By synthesizing obtained contact forces Fx, Fy, Fz of
respective directions, a normal force acting against the probe from
the measured object can be estimated. The probe is allowed to trace
a profile of the measured object with the magnitude of a contact
force F (normal force) of the probe kept constant to measure the
shape. Thus, according to the shape measuring method discussed in
Japanese Patent Application Laid-Open No. 2005-37197, even if the
measured object has a steeply-inclined surface, its shape can be
measured with the contact force applied from the probe to the
measured object kept constant. Accordingly, the shape can be
measured while generation of a system error accompanying an
increase and decrease in the contact force during the measurement
depending on the inclination of the measured object surface is
reduced.
[0006] However, according to the method discussed in Japanese
Patent Application Laid-Open No. 2005-37197, the housing is moved
in the horizontal direction along a fixed scanning track to scan
the probe, and the housing driven by a servo motor is displaced in
the vertical direction. Consequently, a contact force F (normal
force) applied to the probe can be controlled. Thus, when the
measured object surface has a steep slope, for example, a vertical
surface, namely when the displacement direction of the housing for
controlling the force is substantially parallel to the tangent line
of the slope of the measured object, the contact force to the
measured object changes little even if the housing is displaced in
the vertical direction, thereby making it difficult to control the
contact force.
[0007] When the probe is moved in the horizontal direction along
the scanning track and the probe comes into contact with the
vertical surface of the measured object, of a force S urged against
the probe by a movement of the housing, a component force St in the
direction of the tangent to the measured object is so small that
the probe cannot move, thereby making it difficult to scan the
probe along the profile of the measured object.
[0008] Therefore, when the probe is kept in contact with a steep
surface, for example, a vertical surface while controlling the
contact force F applied to the probe by displacing the housing in
the vertical direction, it is difficult to allow the probe tracing
the profile of the measured object, thereby making it difficult to
measure the shape.
SUMMARY OF THE INVENTION
[0009] The present invention is directed to a shape measuring
apparatus and method capable of measuring a shape of a measured
object having a steeply-inclined surface like a vertical plane
while controlling a contact force F applied to a probe by
displacing a probe supporting unit configured to support the probe,
in a vertical direction.
[0010] According to an aspect of the present invention, a shape
measuring apparatus for measuring a shape of a measured object by
scanning a contact type probe along the surface of the measured
object while keeping the contact type probe in contact with the
measured object and measuring a position of the contact type probe,
comprises: a probe supporting unit movable in a 3-dimensional
direction; a contact type probe supported elastically with respect
to the probe supporting unit; a measured unit configured to measure
the position and orientation of the contact type probe; and a
calculation unit configured to calculate a contact force received
by the contact type probe from the measured object based on a
measured position and orientation of the contact type probe,
wherein, the probe supporting unit is configured, when a component
force containing a component in a moving direction of the probe
supporting unit of the contact force exceeds a predetermined
threshold, to be driven in a direction perpendicular to the moving
direction of the probe supporting unit to move the contact type
probe.
[0011] According to another aspect of the present invention, a
shape measuring method for measuring the shape of a measured object
by scanning a contact type probe supported elastically by a probe
supporting unit movable in a 3-dimensional direction along the
surface of the measured object while keeping the contact type probe
in contact with the measured object and measuring a position of the
contact type probe, comprises: measuring the position and
orientation of the contact type probe and calculating a contact
force based on the measured position and orientation of the contact
type probe; scanning the surface of the measured object with the
contact type probe by moving the probe supporting unit while
controlling the contact force to approach a target value with a
force control unit; and driving the probe supporting unit in a
direction perpendicular to the moving direction of the probe
supporting unit to move the contact type probe, when a component
force containing a component in a moving direction of the probe
supporting unit of the contact force exceeds a predetermined
threshold.
[0012] According to the present invention of this application, the
probe supporting unit is driven in a direction perpendicular to a
moving direction of the probe supporting unit based on a contact
force applied to the probe and the magnitude of a component in a
moving direction of the probe supporting unit. Consequently, the
probe can be allowed to trace the surface of a measured object even
if the measured object has a steeply-inclined surface. Therefore,
the shape of even a measured object having a vertical plane can be
measured.
[0013] Further features and aspects of the present invention will
become apparent from the following detailed description of
exemplary embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate exemplary
embodiments, features, and aspects of the invention and, together
with the description, serve to explain the principles of the
invention.
[0015] FIG. 1 is a configuration diagram of a probe according to a
first exemplary embodiment of the present invention.
[0016] FIG. 2 is a configuration diagram of a probe according to a
second exemplary embodiment of the present invention.
[0017] FIGS. 3A and 3B are configuration diagrams of the probe
according to the second exemplary embodiment of the present
invention.
[0018] FIG. 4 is a schematic diagram illustrating the relationship
between a contact force, its component forces and a trajectory
direction of the probe depending on position of a measured object
and a tip ball.
DESCRIPTION OF THE EMBODIMENTS
[0019] Various exemplary embodiments, features, and aspects of the
invention will be described in detail below with reference to the
drawings.
[0020] A first exemplary embodiment will be described below. FIG. 1
illustrates a first exemplary embodiment, indicating the features
of the present invention in detail. Referring to FIG. 1, a shape
measuring apparatus is installed on the floor. Vibration control
bases 2a, 2b are placed on the floor 1 and a measuring base 3 is
provided thereon. The vibration control bases can attenuate
vibration transmitted from the floor 1 to the measuring base. This
measuring base is used for fixing a measured object 4 and three
reference mirrors, and then, this shape measuring apparatus
measures the position of a point on the surface of the measured
object with respect to these reference mirrors.
[0021] The measuring base 3 is a box-shaped construction, in which
the measured object 4 is to be fixed. Further, the measuring base 3
has an X-reference mirror 5 serving as a position reference in the
horizontal direction, a Y-reference mirror (not shown) and a
Z-reference mirror 7 serving as a position reference in the
vertical direction. The measuring base and the reference mirrors
serve as a criterion for measurement and are manufactured with a
material having a small coefficient of linear thermal expansion,
for example, a low thermal expansion ceramic, low thermal expansion
casting iron, low thermal expansion glass. These reference mirrors
serve as position criteria when measuring a distance with a laser
measuring machine.
[0022] A slide configured to move the probe will be described.
Vibration control bases 8a, 8b are installed on the floor 1 and a
scan-axis base 9 is provided thereon. With the scan axis base 9 as
a fixing unit, an X-axis slide 10 movable relatively in the
X-direction in the same Figure and an X-axis motor 11 are provided
on the scan-axis base 9. A Y-axis slide movable relatively in the
Y-direction with respect to the X-axis slide 10 and a Y-axis motor
13 are provided on the X-axis slide 10. Likewise, a Z-axis slide 14
movable relatively in the Z axis with respect to the Y-axis motor
12 and a Z-axis motor 15 are provided on the Y-axis slide 12.
[0023] With this structure, the Z-axis slide 14 is movable
3-dimensionally in the X, Y, and Z directions. A probe supporting
unit 17 is fixed on the Z-axis slide 14 and a probe shaft 19 is
supported by a leaf spring 18 suspended from the probe supporting
unit 17. The leaf spring 18 is constructed of one or a plurality of
thin metal plates, and although the leaf spring 18 is represented
here in a structure of a cantilever beam in FIG. 2, it may be
formed in a double end beam. To secure stability to heat, the probe
shaft 19 is manufactured of a material having a small coefficient
of linear thermal expansion, for example, low thermal expansion
ceramic, low thermal expansion casting iron, low thermal expansion
glass. The probe shaft 19 has a three-sided mirror having a mirror
surface each in the Z direction, X direction and Y direction at its
top end, and a tip ball 21 which comes into contact with the
measured object 4 at its bottom end.
[0024] As described above, the probe supporting unit 17 is fixed on
the X-axis slide, the Y-axis slide and the Z-axis slide, so that
the probe supporting unit 17 is configured to be movable
3-dimensionally. A contact type probe consisted of the probe shaft
19 and the tip ball 21 is supported elastically with respect to
this probe supporting unit.
[0025] A probe small mirror 22 configured to measure a displacement
in the X direction and the Y direction and is provided on the probe
shaft 19 such that it is spaced from the three-sided mirror 20. On
the other hand, the probe supporting unit 17 is provided with an
interferometer configured to measure a displacement of the
three-sided mirror 20 to measure a position and orientation of the
probe. The interferometers include an interferometer Xp1 configured
to measure a displacement in the X direction, and similarly include
an interferometer Yp1 configured to measure a displacement in the Y
direction and an interferometer Zp configured to measure a
displacement in the Z direction (which are not shown). To measure a
distance to the probe small mirror 22, an interferometer Xp2
configured to measure a displacement in the X direction is provided
and, likewise, an interferometer Yp2 configured to measure a
displacement in the Y direction is provided on the probe supporting
unit 17 (which are not shown). A Z-direction distance measurement
small mirror 23 is provide on the probe supporting unit 17 and a
Z-axis interferometer Z1 is provided to measure a distance to the
Z-reference mirror 7. The Z-axis interferometer Z1 and the
interferometer Zp are disposed such that measurement axes thereof
pass through the axis of the probe and the center of the tip ball
21.
[0026] To measure a distance between the X-reference mirror 5 and
the probe supporting unit 17 at two positions, X-distance
measurement small mirrors 24a, 24b are provided on the probe
supporting unit 17. X-axis interferometers X1, X2 configured to
measure these distances are provided on the Z-axis slide 14. As for
the Y direction, Y-axis interferometers Y1, Y2 are provided on the
Z-axis slide 14, although not shown. For these interferometers, a
distance measured by a laser measuring machine is expressed with
the same symbol as the interferometer. For example, a distance
measured by the interferometer X1 is expressed as X1.
[0027] Intervals in the Z direction for installing the
interferometer in each of the X and Y directions are expressed with
following symbols.
L1: interval between the interferometers X1 and X2 (same as an
interval between interferometers Y1 and Y2 (not shown)) L2:
interval between the interferometers X2 and Xp1 (same as an
interval between interferometers Y2 and Yp1 (not shown)) L3:
interval between the interferometers Xp1 and Xp2 (same as
interferometers Yp1 and Yp2 (not shown)) L4: interval between the
interferometer Xp2 and the central position of the probe tip ball
21 (same as an interval between an interferometer Yp2 (not shown)
and the tip ball 21)
[0028] Using measured five distances Xp1, Xp2, Yp1, Yp2, Zp between
the probe supporting unit 17 and each mirror provided on the probe,
central positions Xs, Ys, Zs of the tip ball 21 and rotation angles
Xm, Ym of the probe shaft with respect to the vertical direction
are calculated by a probe position and orientation calculation unit
28 according to following equations.
[0029] In the following equations, it is assumed that Xp1, Xp2,
Yp1, Yp2, Zp indicate distances measured by each interferometer and
indicated with arrows in FIG. 1.
Xs=Xp1+(Xp2-Xp1).times.(L3+L4)/L3 (equation 1)
Ys=Yp1+(Yp2-Yp1).times.(L3+L4)/L3 (equation 2)
Zs=-Zp (equation 3)
Xm=(Xp2-Xp1)/L3 (equation 4)
Ym=(Yp2-Yp1)/L3 (equation 5)
These symbols are determined depending on an installation direction
of the interferometer and how to set up the coordinates. A
measuring unit for measuring the position and orientation of the
above-described contact type probe is constituted by each mirror
and each interferometer.
[0030] Xm represents a ratio of (Xp2-Xp1) indicating a position of
the interferometer Xp2 relative to a position of the interferometer
Xp1, with respect to the length L3, and indicates a tangent tan(Xm)
of an inclination angle Xm of the probe shaft with respect to the
vertical direction. In this case, Xm is a very small value and thus
it is permissible that tan(Xm).apprxeq.Xm. Thus, assuming that a
length of the probe shaft supported by the probe supporting unit
from its rotation center to the tip ball 21 is L, a displacement
.delta.x in the X direction of the tip ball 21 which occurs when an
external force is applied to the tip ball 21 is L.times.Xm.
[0031] The rotation center of the probe can be obtained by
previously measuring which position of the probe shaft is
stationary with respect to an external force on a condition that
the contact type probe is supported elastically on the probe
supporting unit and then an external force is applied to the probe
shaft and the tip ball 21. Thus, using .delta..sub.x,
.delta..sub.y, .delta..sub.z and preliminarily obtained probe
stiffness Kx, Ky, Kz, the probe contact force F is calculated by
the contact force vector calculation unit 27 as follows:
F=((Kx.times..delta..sub.x).sup.2+(Ky.times..delta..sub.y).sup.2+(Kz.tim-
es..delta..sub.z).sup.2).sup.1/2 (equation 6)
The operation of the X-axis motor 11 is controlled by a contact
force control unit 26 to bring the contact force F close to a
predetermined target value Ft to keep the F as constant as possible
to drive the X-axis slide 10. The Y axis and the Z axis are also
controlled in the same way. This is called contact force
control.
[0032] By scanning the probe while performing the contact force
control, the position of the tip ball 21 of the probe tracing the
surface of the measured object 4 can be obtained. If assuming that
the rotation center of the probe never changes during the contact,
the probe small mirror 22 may be omitted and make Xp2 and Yp2 zero.
Assume that L3 and L4 are distances from the rotation center of the
probe up to Xp1 and the probe small mirror 22. Consequently, only
by measuring a displacement of the three-sided mirror 20, the probe
contact force F and the position of the tip ball 21 can be obtained
easily.
[0033] When scanning the probe, a scanning trajectory like, for
example, a raster trajectory, is provided to an X-Y plane by a host
controller 29 to move the X-axis slide 10 and the Y-axis slide 12.
In this case, the X axis and the Y axis are controlled by moving
those slides to a desired position with motors driven by a position
control unit 25. The Z-axis slide 14 is moved by only an action by
the contact force control for controlling F to be constant.
Therefore, to scan the probe while controlling F constant, an
instruction by the position control and an instruction by the
contact force control are added to the X axis and the Y axis by an
X-axis direction movement amount calculation unit 30 and a Y-axis
direction movement amount calculation unit 31, respectively. Then,
the X axis and the Y axis undergo the contact force control and the
position control at the same time to execute the scanning
operation.
[0034] FIG. 4 is a schematic diagram illustrating the relationship
between a contact force F, its component forces Fx and the
trajectory direction of the probe depending on position of the
measured object 4 having a steeply-inclined surface and the probe
tip ball 21. This relationship will be described with reference to
FIG. 4. The probe shaft is omitted from the same Figure to make the
representation easy to understand.
[0035] When the measured object 4 has a surface having a high
inclination angle (e.g., vertical) with respect to the X-Y plane
like a case where the tip ball 21 is located at a position P1 in
FIG. 4 and it is intended to measure the inclined surface, the
surface normal of the measured object 4 and the probe scanning
trajectory become parallel while the directions thereof are
opposite to each other when scanning the probe. In this case, when
the position control and the contact force control are implemented,
the contact force F and a force S generated for the position
control in the scanning trajectory direction cancel each other out
when they are added, thereby disabling scanning the probe on the
measured object at the position P1 in a positive direction of the Z
axis, indicated with a broken line, which the tip ball 21 should
move.
[0036] Even if the surface normal of the measured object 4 and the
scanning trajectory are not completely parallel to each other, if
they are in the vicinity of parallel to each other as seen at a
position P2 in FIG. 4, when the position control and the contact
force control are added, an operation amount generated by a force
St (component force of the force S in a tangent direction with
respect to the measured object 4) applied in the tangent direction
of the measured object 4, which the probe should move, becomes
remarkably small, thereby possibly disabling the probe for
scanning.
[0037] On the other hand, when the probe moves downward on a
steeply-inclined surface along the scanning trajectory of the probe
as indicated at a position P4 in FIG. 4, it is difficult to scan
the probe. When the probe moves downward along the steeply inclined
surface, the force St which the tip ball 21 applies in the tangent
direction of the measured object 4 becomes very small with respect
to the force S. In such a case, there is a possibility that the
probe may float with respect to the measured object, thereby
possibly disabling continuing of measuring of the shape of the
measured object 4 with the contact force F maintained at a
particular value. To solve such a difficulty, according to this
exemplary embodiment, component forces Fx, Fy within the X-Y plane
of the contact force F or a resultant force Fxy thereof is
calculated to monitor a state of the contact force control.
In this case, Fxy is calculated by the contact force vector
calculation unit according to:
Fxy=((Kx.times..delta..sub.x).sup.2+(Ky.times..delta..sub.y).sup.2).sup.-
1/2 (equation 7)
[0038] A case where any one of target values of Fx and Fy is 0 is
equivalent to performing a following control with any one of Fx and
Fy in Fxy. Therefore, by taking a case of controlling Fxy
containing both cases as an example, the shape measuring method of
this exemplary embodiment will be described. As Fxy, which is XY
component of the contact force F, increases and approaches the
value of F, the position control is disturbed along with cancelling
out by the contact force control. Then, for Fxy, a threshold Thxy
is determined with respect to the contact force F and by providing
the Z axis with a velocity Vz in a direction perpendicular to the
probe supporting unit (positive direction of the Z axis in this
exemplary embodiment), which is proportional to a force exceeding
the threshold, the probe is moved upward and the scanning is
continued. The threshold Thxy is determined to be, for example, 0.8
times a target value of the contact force F in the contact force
control. Assuming the proportionality coefficient to be a, Vz is
calculated as follows:
Vz=(Fxy-Thxy).times.a (equation 8)
[0039] As a result, when moving the probe in a direction
perpendicular to the moving direction of the probe supporting unit,
the probe can be prevented from being pressed against the measured
object strongly or conversely, leaving far from the measured
object. In this case, an acceleration Az may be provided to the Z
axis instead of the velocity Vz and the probe supporting unit is
driven in a direction perpendicular to the moving direction of the
probe supporting unit to displace the contact type probe. If the
magnitude of the contact force F is already known, Thxy may be
determined without relating this to the contact force F.
[0040] When a command dispatched in terms of the position control
is appropriate, a position proportional to an integrated value of a
difference between the threshold Thxy and a calculated component
force Fxy may be provided. As a result, the scanning can be
executed while preventing the contact force control and the
position control from cancelling out each other. Thus, even if the
measured object has a steeply inclined surface in the vicinity of
vertical, the probe can be provided with a driving force along the
inclined surface appropriately, thereby enabling the measurement of
the shape.
[0041] FIGS. 3A and 3B are flow charts illustrating an operation of
the probe for measuring the shape of a measured object 4 by
scanning the probe on the surface of the measured object 4 having a
plane nearly perpendicular to the X-Y plane with the probe kept in
contact with the surface.
[0042] First, FIG. 3A is described. In step S1 in FIG. 3A, the tip
ball 21 of the probe is brought into contact with the measured
object 4 and a contact force F generated at that time is obtained
according to the position and orientation of the probe as described
above. The probe is scanned while being controlled such that the
contact force F to approach a target value Ft. This operation
corresponds to controlling the probe pressure to approach a
particular value.
[0043] In step S2, of the contact force F, a component force having
a component in a moving direction of the probe supporting unit,
namely, a scanning direction of the probe is calculated. This step
corresponds to calculating Fxy which is a component in the scanning
direction of the probe out of the contact force F. As evident from
a force applied to the tip ball 21 located at positions P2 and P4
in FIG. 4, the component force has a positive component (P4) or a
negative component (P2) with respect to the scanning direction.
Obviously, the calculated component force does not need to be
parallel to the scanning direction of the probe and any component
force is permitted as long as it has a component in the scanning
direction. The moving direction of the probe supporting unit is
known from a scanning trajectory of the probe, preliminarily
determined before the measurement of the shape for raster scanning
is started.
[0044] In step S3, whether the component force is larger than a
predetermined threshold Th is determined. A case where the
component force (Fxy in the above example) is not larger than a
threshold Th corresponds to a case where the component of a
component force extending in a direction perpendicular to the probe
scanning direction is large. Consequently, the probe can scan along
the surface of the measured object 4 without forces generated by
the position control and the contact force control accompanying the
scanning of the probe cancelling out each other (this case is, for
example, a state at position P3 in FIG. 4). In this case, by
scanning the probe, a component force of the contact force F having
a component in the scanning direction (a moving direction of the
probe supporting unit) of the probe is calculated, and this value
is compared with the threshold Th again.
[0045] In step S4, when the component force is larger than the
threshold Fh, on a predetermined scanning trajectory, the probe
cannot be moved in a direction perpendicular to a current scanning
direction, and thus, the force control of a stylus pressure in the
direction perpendicular to the scanning direction is invalidated.
In this exemplary embodiment, the force control of the stylus
pressure in the Z direction is invalidated. The invalidation of the
force control is stopping the force control and more specifically,
the invalidation can be achieved by turning a gain of an integrator
accommodated in the contact force control unit 26 to zero.
[0046] When the component force Fxy exceeds the threshold Thxy, the
probe supporting unit is driven in a direction perpendicular to the
scanning direction of the probe, for example, in the Z direction to
move the probe. In this case, a phenomenon that the stylus pressure
in the Z direction decreases with a movement of the probe in the Z
direction occurs. The reason is that the probe attempts to stay
there with a friction force. Thus, to move the probe, the force
control to make the stylus pressure in the Z direction constant is
invalidated.
[0047] In step S5, the movement of the probe supporting unit is
canceled to stop the scanning of the probe. In step S6, the probe
supporting unit is driven in a direction perpendicular to the
direction of the scanning trajectory of the probe currently in a
stationary condition, in other words, the moving direction of the
probe supporting unit, to displace the probe. The perpendicular
direction described here is determined depending on which direction
the displacement should be applied to the measured object 4 in
viewpoints of the shape of the measured object 4. In case of a
member having both a semispherical portion and a column portion,
when the tip ball 21 is located at a position P1, a velocity or an
acceleration having a component in the positive direction of the Z
axis is provided to the probe supporting unit 17. Conversely, if a
velocity or an acceleration is provided in the negative direction
of the Z axis, there is a fear that the tip ball 21 of the probe
may collide with a measuring base 3 and it is inconvenient. Because
a schematic shape of the measured object 4 is known in most cases,
it is permissible to preliminarily set in which direction the probe
supporting unit should be moved.
[0048] In step S7, when the component force Fxy becomes smaller
than the threshold Thxy after the probe is moved (YES in step S7),
by "validating" controlling of the probe pressure in the Z
direction to be constant, the probe can return to an operation at
the time of scanning a gentle slope. This operation can be achieved
by "validating" the force control again by setting the gain of the
integrator previously set at 0 to other value than 0.
[0049] In step S8, after that, the scanning of the probe is
restarted and the measurement of the shape of the measured object 4
is restarted. In step S9, whether the scanning of a target area of
the measured object 4 has been terminated is determined and if it
has been terminated (YES in step S10), the measurement is
terminated and otherwise (NO in step S10), the processing returns
to step S1.
[0050] When the probe moves while generating a component force
exceeding the threshold, if the surface of the measured object 4 is
perpendicular to the X-Y plane and then the probe supporting unit
17 continues to operate along the scanning trajectory, it comes
that the probe continues to advance against the surface of the
measured object 4, so that the stylus pressure continues to rise.
Alternatively, when the probe moves downward along a steep slope,
the probe leaves the surface of the measured object 4, so that the
stylus pressure becomes zero and the probe floats in space.
[0051] Then, when the component force Fxy exceeds the threshold
Thxy in the measurement of the shape illustrated in the flow chart
of FIG. 3A, the driving of the probe supporting unit 17 in the
direction of the scanning trajectory (for example, a current
traveling direction for raster scanning) is canceled to stop the
scanning of the probe. When the probe goes beyond the vertical
section, by restarting the scanning with the probe supporting unit
17 in the direction of the scanning trajectory, the scanning with
the probe along the surface of the measured object 4 can be
achieved even if the measured object has a vertical surface.
[0052] When the measured object 4 has no steeply-inclined surface
like a vertical plane, if the component force Fxy exceeds the
threshold Thxy, it is permissible to omit a procedure for stopping
and restarting the scanning operation like the shape measuring
method illustrated in the flow chart of FIG. 3B. The shape
measuring method illustrated in the flow chart of FIG. 3B is the
same as that illustrated in the flow chart of FIG. 3A except that
the steps of stopping and restarting the scanning with the probe
supporting unit 17, indicated in steps S5 and S9 in FIG. 3A are
omitted. This prevents the shape measurement from being stopped
halfway due to erroneous determination and ensures a stabilized
operation.
[0053] Here, it is permissible to apply hysteresis trigger which
changes the value of Thxy between before and after Fxy reaches the
Thxy. In this case, a vibrating action of the probe which occurs
when Fxy changes in excess of Thxy can be blocked to execute a more
stable scanning.
[0054] It is permissible to obtain any component parallel to the
scanning trajectory by calculation instead of Fxy and execute the
scanning the probe using the relation specified in the above
equation 8. In this case, the probe becomes capable of escaping
obliquely into an upward space containing any one of Z and XY by
means of a forced displacement and the contact force control, so
that a more stable scanning can be achieved. However, if a
projection of the scanning trajectory onto the X-Y plane which the
probe travels on becomes different from an initially provided
scanning trajectory, and if it is intended to measure the shape
strictly along the initially provided scanning trajectory, this
method cannot be used.
[0055] Further, by comparing the magnitude of the contact force F
with that of the component force Fxy, an inclination of a measured
object surface which the probe makes contact with can be estimated.
Particularly, if the contact force F and the component force Fxy
are substantially equal to each other, it can be estimated that the
measured object surface is a plane perpendicular to the X-Y plane.
In such a case, by decelerating the scanning velocity in the X-Y
direction of the probe or stopping the probe and then continuing to
move the probe supporting unit 17 in a positive direction of the Z
direction, a phenomenon that the probe advances against the
vertical plane can be avoided. Consequently, even if the measured
object has a steeply-inclined surface, for example, a wall surface
vertical to the X-Y plane, the shape of the measured object can be
measured stably by providing with only the scanning trajectory on
the X-Y plane and without providing any scanning trajectory in the
Z direction.
[0056] A second exemplary embodiment of the present invention will
be described with reference to FIG. 2. Only a fine movement table
16 and a compensation control unit 32, which are not employed in
the first exemplary embodiment, will be described. The fine
movement table 16 is provided on the Z-axis slide 14 such that it
is movable in the X, Y, Z directions. The probe supporting unit 17
is fixed on the fine movement table 16.
[0057] The control of the fine movement table 16 will be described.
The fine movement table 16 which can be inched in the 3-dimensional
direction is related to only the control of the contact force but
not related to the position control of the probe. The contact force
F is kept constant by controlling the fine movement table 16 in the
X, Y, Z directions and a movement of the fine movement table 16 in
the Z direction is compensated by controlling the Z-axis slide 14.
Specifically, an output of the fine movement table 16 is input to
the compensation control unit 32 and the Z-axis motor 15 is
controlled so that the movement amount in the Z direction turns to
zero when moving the Z-axis slide 14. As a result, an effect of
compensating for a stroke of the fine movement table 16 is
generated. This fine movement table 16 may have an effect of
improving a control band of the Z-axis containing the Z-axis slide
14 by employing a table having a driving unit capable of responding
rapidly such as a piezo actuator.
[0058] If the Z-axis control band can be improved in this way, when
a force by the position control and a force by the contact force
control cancel out each other, so that the scanning of the probe is
disabled, the probe can be moved more rapidly. Consequently, if the
position control and the contact force control cancel out each
other when executing a rapid scanning, saturation of the contact
force control can be prevented, thereby achieving a measurement of
the shape having few errors and a small deviation in the contact
force. Further, according to this exemplary embodiment, the contact
force control is carried out by only the fine movement table 16, so
that no influence by the contact force control is applied to the X
and Y slides each. Thus, the probe can be scanned on the measured
object 4 strictly following an initially provided scanning
trajectory.
[0059] Although the first exemplary embodiment and the second
exemplary embodiment have been described above by taking a laser
measuring machine as a unit configured to measure the position and
orientation of the probe like Xp1, the same effect can be secured
even if other measuring unit, for example, a electrostatic
capacitance displacement gauge, an eddy current displacement gauge,
is employed.
[0060] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all modifications, equivalent
structures, and functions.
[0061] This application claims priority from Japanese Patent
Application No. 2011-028716 filed Feb. 14, 2011, which is hereby
incorporated by reference herein in its entirety.
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